1. Industrial Field of the Invention
The present invention relates to a method suitable for manufacturing thick composite articles which require high accuracy in dimensions, especially, high accuracy in plate thickness, and a high internal quality, for example, component parts such as a rotor hub of a rotor system in a helicopter made of composite materials.
2. Description of Related Art
Conventionally, most composite articles which require high strength, such as aircraft parts, have been molded in accordance with an autoclave molding method, as shown in FIG. 8. More specifically, resin-impregnated reinforcing fiber 3 (prepreg) is laid up in a molding jig 2 on a base plate 1. A pressing plate 4 is mounted on the prepreg 3, and covered with release film 5. The molding jig 2 is bagged by a breather 6, a bag film 7 and a vacuum sealant 8 on the base plate 1. After the interior of the bag film 7 is vacuumed, the prepreg 3 which is laminated in the molding jig 2 is heated to a predetermined temperature while an autoclave pressure is applied from the outside of the bag film 7. Thus, the prepreg 3 is molded.
In this autoclave molding method, a uniform pressing force applied to molding articles can be kept constant, so that molded articles of a favorable internal quality can be easily obtained. However, since the plate thickness varies owing to changes in the fiber area weight and/or the resin content of the material and so forth, high accuracy in the plate thickness can not be obtained constantly especially in the case of thick articles whose ply number is large. Therefore, this method is not suitable for component parts of a rotor system or the like which require highly accurate outer dimensions.
On the other hand, in order to obtain high accuracy in dimensions, molding is performed in accordance with a conventional matched-die method, as shown in FIG. 9. In this method, a sufficient pressure will not be uniformly applied to prepreg 3 after closing dies 48, so that voids 9 will be easily formed. Consequently, this method is not suitable for component parts of a rotor system or the like which require high strength properties.
In addition, as shown in FIG. 10, there is an RTM (Resin Transfer Molding) method in which a preform 10 of reinforcing fiber is set in a molding die 11, and resin 12 is injected into the molding die 11 under a pressure, to thereby obtain a molded article.
In this RTM method, it is necessary to inject the resin 12 in the molding die 11 so as to impregnate the preform 10 with the resin 12 uniformly. Therefore, as shown in FIG. 10, this method requires a complex injection apparatus comprising a pressing valve 13, an injection cylinder 14, a resin supply line 15, a resin draining line 16, a vacuuming line 17 and a vacuum pump 18.
Further, the kinds of resin employed in the RTM method are limited. More specifically, resin of low viscosity is preferable with respect to impregnation efficiency. However, the resin of low viscosity tends to be inferior in toughness, so that its products will be brittle. On the other hand, resin with favorable toughness tends to have high viscosity. Consequently, it easily induces inferior impregnation, and also, a high resin injection pressure is required, which results in a problem, for example, the regularity of the fiber is easily disarranged at the time of injection. This is particularly disadvantageous for parts which require a high fiber volume content (Vf).
Moreover, in the RTM method, it is difficult to produce a reinforcing fiber preform of a complicated shape and a layer structure. In other words, the RTM requires means for efficiently preparing a reinforcing fiber preform which includes reinforcing fiber arranged to be of a predetermined shape and thickness and in a predetermined fiber direction, and which will have less deformation, fiber disarrangement and fraying while it is handled. Such a preform for a part of a large complex shape and a complicated laminated structure, e.g., a rotor hub of a helicopter, cannot be prepared without difficulty.
Thus, the RTM method is not suitable for parts which are large, thick and complicated in shape, and which require high strength properties, such as a rotor hub of a helicopter.